Facebook's stock: Insiders can now start selling (pictures)

Thiel made a big bet on Mark Zuckerberg in 2004 when he put $500,000 into Facebook, becoming one of the first outside investors in the social network. He cashed out $640 million in the IPO -- at a price more than 40 percent above where the stock trades now -- and it's never easy to sell a stock when it was worth so much more just a few months ago. We wouldn't expect him to immediately dump his holdings, but who knows. He could gradually start letting some go.

Breyer, with Accel Partners, courted Zuckerberg hard and eventually got him to agree to take a $12.7 million investment that valued the company at $93 million. What a bet that turned out to be. Accel already sold $2.1 billion worth of stock into the IPO, so selling more soon doesn't seem like an obvious move. It still owns 35.7 million Class A shares, and 7.9 million Class B shares.

Bono is a partner at Elevation Partners, a Silicon Valley venture firm that's had its share of struggles, such as its investment in Palm. And then there's its Facebook bet, which it made in 2009. Elevation sold $176 million worth of stock at the IPO but still has 35.5 million Class B shares that it might be eager to get rid of.

Steve Ballmer tried to buy Facebook outright in 2007 for $15 billion, but Mark Zuckerberg wasn't interested. Instead, Microsoft's CEO scored a deal in which Microsoft invested $240 million for a 1.6 percent stake. Microsoft cashed in $249 million at the IPO but still holds 26.2 million Class B shares. The company wants to do more, not less, with Facebook, so it seems unlikely that cash-rich Microsoft will start selling.

The Russian tycoon Yuri Milner wooed Zuckerberg in 2009 and made an investment that's already paid off big time. He's cashed out more than $1.7 billion at the IPO, and might be looking to shed some more, especially considering that two of his other big bets -- Zynga and Groupon -- are cratering. His investment firm, DST Global, still owns 80.6 million Class B shares, and 5 million Class A.

Pincus, the CEO of Zynga, was an angel investor in Facebook, having put up $40,000 alongside investments by Peter Thiel and Reid Hoffman. He recently took some stock off the table -- to the tune of $38.9 million -- but he still owns 4.3 million Class B shares. And Pincus has been in a selling mood of late. He's being sued for selling shares of Zynga at $12 a share just before the stock started to tank. It wouldn't be surprising to see Pincus dump more Facebook stock, and fast.

The LinkedIn founder, who invested in Zuckerberg early on, put in only $40,000, partly because he was building his own social network. He cashed out $36 million at the IPO but still sits on 3.8 million Class B shares. He seems unlikely to run for the exit.